Abstract The parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor (PTHR1) plays vital roles in bone development and in maintaining calcium and phosphate homeostasis. The PTHR1 mediates these roles by binding two distinct polypeptide hormone ligands - PTHrP and PTH. The PTHR1 is of high medical importance as it is associated with diseases such as osteoporosis, hypoparathyroidism and Jansen's metaphyseal chondrodysplasia. This makes the PTHR1 an important target for therapeutic interventions. Developing effective drugs for the PTHR1 is challenging, however, especially by rational design approaches. The few drugs currently available for the PTHR1 are not ideal. For osteoporosis, the two available drugs: recombinant PTH(1-34), forteo, and a synthetic PTHrP(1-34) analog, abaloparatide, improve bone parameters but utility is limited by factors such as the need for daily injection, as the peptides would be destroyed by gut proteases if taken orally, the potential for adverse effects, such as excess bone resorption and hypercalcemia, and a waning of anabolic efficacy over time. A better understanding of the molecular mechanisms by which ligands bind to the PTHR1 and activate signaling is therefore needed to facilitate the design of more effective PTHR1-based therapies, particularly ones that are orally active. The studies proposed here aim to reveal such new aspects of PTHR1 mechanisms, and to thus define how peptides as well as small-molecules bind to this G protein-coupled receptor and induce selective signaling responses in target cells that ultimately lead to improvements in human health. !